1. Field of the Invention
This invention generally relates to a voltage controlled oscillator, and more particularly to a multiphase voltage controlled oscillator.
2. Description of the Related Art
Recently, multiphase voltage controlled oscillators (VCOs) have been widely applied to data-recovery systems and phase locked loop (PLL) circuits. A multiphase voltage controlled oscillator can output a plurality of oscillating signals, which have the same frequency but different phases, by controlling a voltage applied at its input, wherein the frequency of each oscillating signal is proportional to the voltage. Ideally, the phases of the oscillating signals outputted from the multiphase voltage controlled oscillator are spread uniformly throughout a range from 0 to 360 degrees so as to achieve an optimal control in applications.
A typical multiphase VCO is generally formed by N delay elements as shown in FIG. 1. The multiphase VCO 100 comprises N differential delay elements 102 serially connected to one another. Each differential delay element 102 has two inputs 102a, 102b and two outputs 102c, 102d, and is serially connected to one another so as to form a ring-type oscillator, such that the outputs 102c, 102d of all of the delay elements 102 can output a plurality of oscillating signals, which have the same frequency but different phases. In such a configuration, the number of the generated oscillating signals is determined by the number of the delay elements. Accordingly, if, for example, 32 oscillating signals are desired to be generated by the multiphase VCO 100, 16 differential delay elements 102 are required.
However, since each delay element has a predetermined signal delay time according to its electrical characteristics, the phase differences among the oscillating signals generated by the multiphase VCO 100 may be confined by the length of the predetermined signal delay time, such that even smaller phase differences cannot be obtained.
Generally, the conventional multiphase VCO adopts a phase interpolation approach in order to obtain smaller phase differences. Taking the multiphase VCO 100 shown in FIG. 1 for example, the phase interpolation approach can produce an extra oscillating signal between every two adjacent oscillating signals. Therefore, the multiphase VCO 110 using such a phase interpolation approach can generate more oscillating signals (i.e. more than 32) from the same 16 differential delay elements 102.
However, the uniformity of the phase differences generated through the phase interpolation approach is easily affected by manufacturing process variation or temperature fluctuation, and thus is difficult to achieve during high frequency and low frequency operations.